Furnace Repair in San Diego: Diagnosed Before It Is Fixed

San Diego furnaces sit idle for eight or nine months, then get asked to work on a 45°F morning in December. That pattern causes a specific set of failures, most of them in the ignition and safety chain rather than the burners themselves. In order of how often we find them:

• Hot-surface igniter. A brittle ceramic element that glows to light the gas. It cracks with age or a hard start, and the furnace goes through its startup sequence without ever lighting.
• Flame sensor. A small rod that proves the burner is lit. A film of oxidation makes the control think there is no flame, so it shuts the gas off after a few seconds. The furnace lights, runs briefly, and stops, three times, then locks out.
• High-limit switch. Shuts the burners off if the furnace overheats. It trips when a filter is clogged, registers are closed, or the blower is weak. A tripping limit is a symptom; the airflow problem underneath it is the repair.
• Inducer motor and pressure switch. The inducer pulls exhaust through the heat exchanger before ignition; the pressure switch proves it. A seized or noisy inducer, a cracked hose or a blocked flue stops the furnace from lighting at all.
• Blower motor and capacitor. A humming furnace with no air, or heat that comes on and then trips the limit, is usually the capacitor or the blower motor. On older PSC motors, the capacitor is a cheap fix; ECM motors cost more.
• Control board. Diagnostic LED blinking a fault code, relays that click without doing anything, or a furnace that runs the fan constantly. Boards fail from heat, moisture in attic installations and power surges.
• Thermostat and wiring. A dead thermostat battery, a loose W wire, or a heat pump thermostat wired for the wrong reversing-valve logic (O vs. B) will keep a perfectly good furnace from ever getting the call. See thermostats.
• Gas valve. The least common and the one we take most seriously. A valve that will not open, or worse, will not close fully, is replaced, never adjusted.

Safety first: carbon monoxide and gas

A furnace burns gas inside your house. Two situations should stop you from waiting for a repair appointment:

• You smell gas. Leave the house, leave doors open behind you, do not flip switches, and call SDG&E's gas emergency line, 1-800-411-7343, or 911, from outside. SDG&E responds to gas leaks around the clock at no charge. Call us after the gas is made safe to repair whatever caused it.
• Your CO alarm sounds, or you have headaches and nausea that clear when you leave the house. Turn the furnace off at the thermostat and the switch on the furnace, open windows, and get out. A cracked heat exchanger, a blocked flue or a disconnected vent pipe can all put carbon monoxide into the supply air. Any furnace that has done this stays off until it is inspected.

Every furnace repair visit includes a look at the heat exchanger, the flue connection and the flame pattern. If we find a crack, we show it to you and the furnace is tagged off. That is a replacement conversation, not a repair.

What to check before you call

• Thermostat set to Heat and above room temperature; fresh batteries if it has them.
• Furnace power switch (looks like a light switch on or near the furnace) on; breaker not tripped.
• Filter: if it is gray and packed, replace it and give the furnace ten minutes to cool before trying again.
• Furnace door panel seated. Most furnaces have a door switch that cuts power when the panel is off.
• Condensing furnace: condensate drain not clogged (a full trap will stop ignition).

If the furnace still will not heat, note what it does when it tries (clicks, hums, lights and quits, nothing at all). That detail shortens the diagnosis.